1. Field of the Invention
The present invention relates to a method of printing on a thermal medium by aligning a test pattern. More particularly, the present invention relates to a method of printing on a thermal medium by aligning print starting positions at a first surface and a second surface of the medium, which is used in a thermal printer.
2. Description of the Related Art
A thermal printer can be divided into a type of printer that uses a medium that represents a predetermined color by responding to heat (hereinafter, referred to as thermal medium), and a type of printer that uses an ink ribbon that transfers a predetermined color onto a general medium responding to the heat in order to print images on the general medium. The ink ribbon type of printer uses a driving device for operating the ink ribbon, thus it has a more complex structure and a correspondingly higher price. Also, the ink ribbon needs periodic replacement, which increases the per page printing price.
Referring to FIG. 1, a thermal medium 10 includes a base sheet 11 having two surfaces, that is, a first surface 10a and a second surface 10b, on which ink layers of predetermined colors are respectively formed. The ink layers are formed to have different colors from each other. For example, a yellow (Y) layer and a magenta (M) layer are sequentially stacked on the first surface 10a, and a cyan (C) layer is formed on the second surface 10b. It is desirable that the base sheet 11 is formed of a transparent material. Reference numeral 13 is a reflective layer that reflects light so that a color image can be seen on the first surface 10a. An example of the thermal medium 10 is disclosed in U.S. Pat. No. 6,801,233, which is assigned to the Polaroid Corporation, the entire contents of which are incorporated herein by reference.
The thermal printer using the thermal medium 10 uses a thermal printhead (TPH), in which heating elements are disposed perpendicular to the direction in which the printing sheet is fed. To perform dual-surface printing using one TPH, the printing process for the first surface 10a of the medium 10 is performed, and then, the printing process for the second surface 10b of the medium 10 is performed again using the same TPH. When the two surfaces are printed, a color image can be seen on the first surface 10a. 
FIG. 2 is a view illustrating a structure of a conventional thermal printer. Referring to FIG. 2, the thermal printer includes a feeding roller 2 that conveys the thermal medium 10, a platen 3 supporting a surface of the medium 10, and a TPH 4 forming an image on the medium 10 that is disposed on the platen 3. A printer having one TPH 4 typically prints on both surfaces of the medium 10 in sequential order by rotating the medium 10 or the TPH 4. Reference numeral 5 is an idle roller that pushes the medium 10 that passes between the idle roller 5 and the feeding roller 2 toward the feeding roller 2.
In the case where the TPH is not aligned with the medium when the TPH is rotated for printing images on the second surface after printing images on the first surface, the color printing operation can produce misaligned printed images on the second surface.
Therefore, a method of aligning a print starting position of the medium is required when the first and second surfaces of the medium are printed.